Exhaust aftertreatment systems are used to remove undesirable emissions from the exhaust of fossil fuel powered systems (e.g. diesel engine, gas engines, gas turbines), which may be used to drive, for example, generators, commercial vehicles, machines, ships, and locomotives. Exhaust aftertreatment systems may include a variety of emissions treatment technologies, such as diesel oxidation catalysts (DOCs), diesel particulate filters (DPFs), selective catalytic reduction catalysts (SCRs), lean NOx traps (LNTs) or other devices used to treat the exhaust.
Selective Catalytic Reduction (SCR) systems provide a method for removing nitrogen oxide (NOx) emissions from fossil fuel powered systems. During SCR, a catalyst facilitates a reaction between a reductant and NOx to produce water and nitrogen gas, thereby removing NOx from the exhaust gas. Generally, the reductant is mixed with the exhaust upstream of the SCR catalyst.
A commonly used reductant in the SCR process is aqueous urea (i.e. a water-urea mixture) because, unlike ammonia, urea is non-toxic and easy to handle, while also being easy to transport, inexpensive and commonly available. Typically, to facilitate SCR, aqueous urea is injected into the exhaust stream of an engine prior to an SCR catalyst. The heat of the exhaust causes the aqueous urea to evaporate and decompose forming ammonia, which will react to reduce the NOx. Liquid urea injection systems, however, introduce added complexity and cost into the process. Thus, alternative methods of utilizing aqueous urea to introduce ammonia into an exhaust system have been sought.
U.S. Pat. No. 7,273,595, by Spokoyny. (hereinafter the '595 patent), discloses such a system. The '595 application discloses passing aqueous urea through or in the vicinity of an ultrasonic transducer to produce a powerful sonic field in the urea. The sonic field causes acoustic cavitation, which in turn causes ammonia and carbon dioxide to be generated and released from the solution. A blower is then used to blow the gaseous ammonia byproduct, along with dilution air, to a location for introduction into an exhaust stream.
While the system disclosed in the '595 application may be suitable to produce ammonia from urea without the need for a liquid injection system, the system and method may be unsuitable for applications that require more than small amounts of urea. Furthermore, the disclosed system introduces additional cost and complexity by requiring the addition of an ultrasonic transducer for generation of gaseous ammonia and a blower for transport of the gaseous ammonia.